<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            width: 500px;
            height: 300px;
            background-color: palegreen;
            display: flex;
            flex-direction: column;
            align-items: center;

        }
        .b{
            width: 20px;
            height: 20px;
            border: 1px solid black;
            margin-bottom: 10px;
        }
    </style>
</head>
<body>
    <div class="box">
        <div class="b"></div>
        <div class="b"></div>
        <div class="b"></div>
     </div>
</body>
</html>